The present invention relates to satellite communication systems, and more particularly to a system for use with a mobile platform having an antenna aperture performing receive and transmit functions, and to a method for causing the antenna aperture to accurately track a target satellite while the mobile platform is moving.
High precision tracking of communication satellites from mobile platforms such as aircraft, ships and land vehicles is required for both optimizing data rate (i.e., peak energy from/to target) and for preventing interference with satellites orbiting adjacent to a target satellite. Various methods for tracking have been used including xe2x80x9cdead-reckoningxe2x80x9d or open-loop, where a calculation is made as to the correct pointing angles based on known satellite and platform positions and platform attitude. Other methods, such as xe2x80x9cclosed loopxe2x80x9d tracking methods, make use of some form of feedback control by signal optimization. These methods work adequately for received signals but not adequately for transmitted signals from the mobile platform to satellites over distances which create latency in the feedback control loop with respect to the required bandwidth. Such a situation would be represented by, for example, geostationary satellites communicating with mobile platforms having high attitude accelerations, such as aircraft and land vehicles.
For centering an antenna aperture with respect to a received antenna beam from a satellite, one well known technique is sequential lobing. Sequential lobing involves steering the antenna aperture deliberately a known distance away from a peak received signal in each of four directions around the peak signal. This is illustrated in FIG. 1. Measurements of the received power or decoded signal are used to calculate where the actual peak of the beam or signal is located, as indicated in FIG. 1b, and the beam is recentered for the next cycle. A receive-only antenna can use this approach quite effectively to maintain pointing accuracy. However, the deficiency with this approach is that a transmitting antenna slaved to such a receive beam will execute the same steptracking, inherently adding inaccuracy as the beam is deliberately stepped away from the known center.
A second technique of antenna pointing relies on increasingly accurate methods of open-loop calculation to maintain accurate pointing. Even if arbitrarily accurate tracking during movement of the mobile platform is possible, the major drawback with this approach is in initially establishing an accurate starting point (i.e., initially determining an accurate estimate of the position of the target satellite relative to the mobile platform).
Prior systems have separated the receive and transmit functions into discrete apertures so that sequential lobing can be performed with the receive beam, and then the transmit beam is computationally slaved to the correct position. This is illustrated in FIG. 2.
For antennas in which transmit and receive functions are performed in the same physical antenna aperture, several error sources are eliminated, but the technique of sequential lobing cannot be used when the transmit beam pointing accuracy requirement is smaller than the step-size of the misalignments used during the sequential lobing process. Such a situation would still result in an error (i.e., misalignment) in the pointing angle of the transmit antenna relative to the target satellite. This is shown in FIG. 3.
Therefore, there is a need for a method of accurately pointing an antenna which performs both receive and transmit functions, and which is located on a moving platform, at a target satellite. More specifically, there is needed a method which allows an antenna aperture performing both transmit and receive functions to initially acquire a signal from a target satellite, and thereafter to accurately determine the beam center of a receive beam transmitted by the satellite, to use an inertial reference unit (IRU) of the mobile platform for tracking the satellite, and to periodically xe2x80x9cfine adjustxe2x80x9d the pointing of the antenna aperture without significantly impeding the transmission of data from the mobile platform to the satellite.
The present invention is directed to a method for more accurately tracking and communicating with a satellite from a mobile platform. The method of the present invention is particularly adapted for use with antenna apertures which are required to receive and transmit information to and from a target satellite while the mobile platform which the antenna aperture is mounted on is moving.
The method of the present invention involves initially requiring the satellite to use the mobile platform""s inertial reference unit (IRU) and information stored in an antenna controller associated with the antenna aperture as to the approximate location of the target satellite. Once the signal from the satellite is acquired, a conventional sequential lobing process is performed to more accurately center the antenna aperture relative to the receive beam received by the aperture. Once the sequential lobing process is completed, the antenna can then be used to transmit data or other information from the mobile platform to the satellite. While transmitting, a second inertial reference unit in the form of a xe2x80x9crate gyroxe2x80x9d local to the antenna is used instead of the IRU of the aircraft to maintain the antenna aperture pointed at the target satellite.
Periodically the antenna controller interrupts the transmission of data or other information from the antenna to the target satellite and again uses the receive capability of the antenna to perform the sequential lobing process. Once this process is completed, transmissions from the antenna are again enabled, thus allowing the antenna to be used to transmit information to the satellite. This process of alternately transmitting from the antenna aperture and inhibiting transmissions while performing the sequential lobing process is repeated continuously to eliminate inertial reference drift error which could otherwise eventually accumulate to an unacceptable level.
The above-described method does not significantly interfere with the transmission of data or other information from the antenna because the sequential lobing process can be performed in a matter of milliseconds.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.